1. Field of the Invention
The present invention generally relates to a wavelength detecting apparatus and a laser apparatus containing such a wavelength detecting apparatus. More specifically, the present invention is directed to a wavelength detecting apparatus and a laser apparatus, capable of measuring and controlling a wavelength of a laser beam or the like to be outputted. Further, the present invention concerns a method for detecting a wavelength by employing such a wavelength detecting apparatus.
2. Description of a Related Art
Excimer lasers have been used as light sources of reduction-projection exposing apparatus (will be referred to as “stepper” hereinafter) for manufacturing semiconductor devices. Only synthetic quartz and fluorite (calcium fluoride) can be used as lens materials capable of sufficiently highly transmitting wavelengths (for example, 248.4 nm and 193 nm) of KrF and ArF excimer lasers, and also having better uniformity and high processing precision. Therefore, it is practically difficult to design reduction-projection lenses in which chromatic aberration has been corrected under spontaneous oscillating line widths (350 pm to 400 pm) of excimer lasers.
As a consequence, in the case where a laser beam having short wavelengths is employed as light sources of steppers, bandwidths of the output laser beam must be narrowed up to such line widths (approximately 0.6 pm) under which chromatic aberration is negligible. Moreover, wavelengths of the output laser beam whose bandwidth has been narrowed must be controlled under highly stable condition.
The stabilization of oscillating wavelengths of excimer lasers whose bandwidths had been narrowed is carried out in such a manner that a portion of excimer laser output light is entered into a spectroscope and is measured by employing either an etalon or a grating. The techniques for detecting (measuring) oscillating wavelengths of excimer lasers are disclosed in Japanese Patent Application Laid-open JP-A-11-298084 and JP-A-6-188502. In these publications, an etalon, which corresponds to a spectroscopic element formed based upon the theory of a Fabry-Perot interferometer, is employed so as to measure oscillating wavelengths of lasers.
FIG. 8 shows a construction of a general-purpose etalon. As shown in FIG. 8, the etalon 6 owns such a structure that two optical substrates 2, on which partial reflection films 1 are formed, are arranged via a spacer 3 in such a manner that planes of these partial reflection films 1 are located opposite to each other, while a predetermined air gap is maintained. The thickness of this spacer 3 is precisely managed. Each of the optical substrates 2 is fixed inside a protection housing 5 via an adhesive agent layer 4 formed by using adhesive agent having proper flexible, holding force, and stable characteristics. In this case, the spacer 3 may be properly formed by ceramics having less thermal expansion characteristics, for example, “Zerodur (registered trademark)”. Alternatively, the etalon 6 may be fixed by a spring instead of the adhesive agent.
A wavelength measurement by the etalon 6 is carried out based upon the below-mentioned basic formula (1) of the Fabry-Perot interferometer.mλ=2nd·cos θ  (1)where symbol “m” represents an order (integer), symbol “d” represents a length of the air gap, symbol “n” represents a refractive index of a gaseous body between the air gaps, and also, symbol “θ” represents an angle (fringe angle) defined between a normal line of the partial reflection film plane of the etalon 6 and an optical axis of incident light.
Light entered into the etalon 6 forms a fringe waveform. FIG. 9A is a diagram for representing a basic idea as to forming of a fringe waveform, and FIG. 9B is a diagram for representing a relationship between a fringe waveform detected by a sensor and optical intensity thereof.
In FIG. 9A, light which has transmitted the etalon 6 where two sheets of optical substrates 2 are arranged in parallel to each other is focused by a lens 7 at a position of a focal distance “f” of this lens 7, and then this light forms a fringe waveform 8. At this time, when both a wavelength “λ” of this light and a fringe angle “θ” satisfy the above-mentioned basic formula (1) of the Fabry-Perot interferometer, the light can pass through the etalon 6 under such a condition that phases of light reflected in a multiple mode within this etalon 6 are made coincident with each other, and then can be collected by the lens 7 at a specific position. Although only one incident light “L” is illustrated in FIG. 9A, since light penetrated through a scattering plate is traveled along various directions, a collected light image of such light capable of satisfying the above-mentioned formula (1) constitutes a plurality of coaxial circles, as indicated in FIG. 9A. When a photodetector is set at the focal position of the lens where these coaxial circles are formed, the photodetector detects strong optical intensity at the positions of the coaxial circles as shown in FIG. 9B, and thus can acquire a fringe waveform 9 that is a coaxial-shaped strong/weak pattern (interference fringe). This fringe waveform 9 is imaged by a semiconductor image sensor such as a CCD. Since positions of the imaged fringe waveform 9 as shown in FIG. 9B are detected, a fringe radius “r”, is calculated, and then, the fringe radius “r” is converted into the fringe angle “θ” based upon a formula: θ=cos−1 (r/f). Thus, the wavelength “λ” of the incident light “L” may be detected by employing the above-mentioned formula (1).
Since the refractive index “n” of the gas existed between the air gaps of the etalon 6 is changed depending upon density distributions of the gas, which are caused by temperature distributions and pressure distributions of this gas, the fringe angle “θ” with respect to the same wavelength “λ”, is changed, and thus, measurement results of wavelengths are varied in connection with a time elapse. Accordingly, when a precise measurement is required, calibration must be regularly carried out. For instance, Japanese Patent Application Laid-open JP-A-11-298084 discloses such a technique for performing calibration while absorption lines of atoms caused by platinum vapor and iron vapor are used as reference wavelengths. Also, Japanese Patent Application Laid-open JP-A-6-188502 describes another technique for performing calibration while an emitted ray of a low-pressure mercury lamp is employed as a reference wavelength. However, in the calibration technique disclosed in Japanese Patent Application Laid-open JP-A-11-298084, since the wavelength of the laser beam must be scanned so as to search the reference wavelength in the calibration, the downtime condition occurs, under which the laser cannot be utilized as the light source of the semiconductor manufacturing stepper, during the calibration. As a consequence, the above-explained calibration cannot be performed other than a limited time duration such that, for instance, the calibration may be limitedly performed one time when laser gas is replaced (every 1 week).
In order to eliminate the calibration (especially, calibration requiring downtime) which is required in connection with changes in peripheral environments (temperatures and pressure) of etalons, or in order to prolong at least execution intervals of calibration, the following idea is conceivable. That is, etalons may be arranged inside, for example, a housing having an airtight characteristic, into which stable gas such as dry nitrogen gas is filled. With employment of such a structure, variations of the refractive index “n” of the gas between the air gaps may be reduced. Alternatively, peripheral temperatures of the housing may be made constant. Also, while temperatures and pressure inside the housing are measured by employing sensors, measured wavelengths may be corrected based upon the refractive index “n” of the gas between the air gaps of the etalon and the thermal expansion of the spacers. These techniques are disclosed in, for instance, Japanese Patent Application Disclosure JP-A-10-506232, which corresponds to WO 96107224, and Japanese Patent Application Laid-open No. 2000-136964.
However, the following fact can be revealed. That is, drifts in measured wavelengths of etalons are conducted not only by the refractive index “n” of the gas between the air gaps and the length variation of the air gaps, but also other serious factors.
Normally, a partial reflection film formed on one plane of an etalon is formed by way of a multi-layer coating in which substances having different refractive indexes are alternately coated (so-called “multi-layer thin-film coating”). In the case of such an etalon which measures ultraviolet light such as an excimer laser beam and a fluorine molecular laser beam, a multi-layer thin-film made of either an oxide film or a fluoride film is formed by which less ultraviolet light is absorbed. Among these multi-layer thin-films, it is known that characteristics (refractive index, spectroscopic transmittance, thickness etc.) of a certain thin-film are changed by highly responding to moisture or vapor of peripheral environments in connection with absorption or desiccation of the moisture. This characteristic change may shift the fringe angle “θ” and may cause errors in wavelength measuring operations. An adverse influence caused by the shift of the fringe angle “θ” due to the characteristic change of this partial reflection film has not been considered when wavelengths of incident light were calculated by employing the basic formula (1) of the wavelength measurement by the etalons.
As a consequence, even when an error caused by temperature and pressure changes between air gaps is continuously corrected based upon the basic formula (1) of a wavelength measurement by an etalon in an interval of regular calibration of the etalon, a fringe angle “θ” caused by characteristic changes in water absorption or desiccation of a partial reflection film is shifted and an error of the wavelength measurement is increased.
The calibration formula employed in the wavelength calibration in Japanese Patent Application Laid-open JP-A-11-292084 is given as follows:λ=λ0+Cd(r2−r02)+N·FSR  (2)In this formula (2), symbol “λ” represents a wavelength corresponding to the fringe radius “r”; symbol “λ0” represents a wavelength corresponding to another fringe radius “r0” and constitutes a reference of calibration; symbol “Cd” represents a constant depending upon optical design; symbol “FSR” represents a free spectrum range of the etalon; and also symbol “N” represents an integer.
In accordance with this publication, when an absolute wavelength is calibrated, an oscillating wavelength of a narrow-band-oscillated excimer laser is scanned, and light intensity of absorption lines given by platinum vapor in a platinum bubble cell is monitored. In this case, at the same time, the radius “r” of the fringe waveform is monitored, and such a radius is stored as the radius “r0” of the fringe waveform at the wavelength “λ0” when the light intensity absorbed by the platinum bubble cell becomes maximum, so that the absolute wavelength calibration is carried out in a regular manner.
Also, the calibration formula employed in the wavelength calibration in Japanese Patent Application Laid-open JP-A-6-188502 is given as follows:λ=λe+FSR(r2−r02)/C  (3)In this formula (3), symbol “λ” represents a wavelength corresponding to the fringe radius “r”; “λe” represents a wavelength of light to be detected when a fringe radius of reference light of calibration is coincident with a fringe radius of the light to be detected; symbol “r0” represents a fringe radius of the reference light; symbol “C” represents a constant depending upon optical design of the etalon; and symbol “FSR” represents a free spectrum range of the etalon.
In accordance with this publication, instead of the narrow-band-oscillated excimer laser, the oscillated light of the mercury lamp is employed so as to detect the. radius “r0” of the fringe waveform of the etalon. Also, the constant “C” is calculated from the detected fringe waveform. Thus, the fringe radius “r0” of the reference light and the constant “C” in the formula (3) are adjusted.
However, both the formulae (2) and (3) are introduced based upon the basic formula (1) of the Fabry-Perot interferometer. If the fringe angle “θ” is shifted in connection with the water absorption or the desiccation of the partial reflection film, then these values of r, r0, FSR, Cd, λe and C in the formula (2) or (3) are changed. For instance, in the formula (3), since the refractive index “n” of the gas between the air gaps is changed in response to the environmental change, the adjustment of the values “r0” and “C” is regularly carried out. However, in the case where the wavelength of the reference light is largely separated from the wavelength of the excimer laser (for example, 253.7 nm of mercury lamp and 193 nm of ArF), the shift in the fringe angle “θ” in connection with the water absorption or the desiccation of the partial reflection film of the etalon may essentially change the wavelength “λe” to be detected in the etalon spectroscope, so that measurement errors of wavelengths are conducted. This phenomenon is conceived as follows. That is, a relationship between the wavelength of the light source which constitutes the reference of calibration and the optical path difference of the partial reflection films of the etalon in each of the wavelengths of the laser beam to be monitored is changed due to the water absorption or the desiccation of the film.
Similarly, also in the formula (2), the value FSR is handled as a substantially constant value with respect to variations caused by the environmental change as to the refractive index “n” of the gas between the air gaps. However, since the optical path length between the partial reflection mirrors of the etalon is changed due to the water absorption or the desiccation of the partial reflection film of the etalon, the fringe angle “θ” is shifted, which may cause the measurement error of the wavelength.
When air contained in the housing, where the etalon is arranged, is replaced with dry nitrogen gas, an increase of gas density inside the housing may be observed just after the gas replacement has been accomplished. It should also be noted that this expression “gas density” implies density of all sorts of gaseous bodies containing vapor inside the housing. This increase of gas density is caused due to the following reasons. That is, moisture or the like, which have been present from the beginning or absorbed on the wall surfaces of the housing, especially plated materials or optical components arranged inside the housing, is gasified (namely, out gas is produced) and then the gaseous moisture are released into the dry nitrogen gas. In this case, moisture absorbed in the partial reflection film of the etalon is released into the dry nitrogen gas for a few minutes after sealing. To the contrary, when the amount of moisture released into the dry nitrogen gas is increased, such a phenomenon will occur. That is, a portion of the moisture contained in the dry nitrogen gas is very slowly reabsorbed into the partial reflection film of the etalon. In these stages, the above-mentioned sifts of the fringe angle “θ” may occur in connection with the water absorption or the desiccation of the partial reflection film of the etalon. Also, shifts of the fringe angle “θ” may similarly occur even when a laser beam is irradiated. In other words, moisture which has been absorbed into the partial reflection film of the etalon is released into the dry nitrogen gas when the laser beam is irradiated thereto and the released moisture is reabsorbed by the partial reflection film when the irradiation of this laser beam is ceased, thereby the shift of the fringe angle “θ” occurs.
In the case of executing, for example, calibration just after gas replacement which calibration is accompanied with downtime and can be carried out only once per one week within a limited time, wavelength measurement errors are increased between calibration executing works, which may cause a high precision measurement to be hardly realized.
Also, after internal gas components of the housing where the etalon is arranged have been replaced with dry nitrogen gas, substances other than moisture are gasified from the plated materials, the partial reflection films and so on, so that so-called “out gas” is exhausted. This “out gas” corresponds to moisture and other substances which have been present or absorbed in the wall surfaces of the housing and the optical components provided in this housing. As the substances other than the moisture, for example, the following substances may be conceived, namely, volatile substances derived from adhesive agent used in the housing, processing oil left on machine processing components, and cleaning alcohol used when the housing is assembled. Also, if such strong ultraviolet rays as the excimer laser beams are irradiated onto the optical components employed in the housing, then it is so conceived that gas is produced due to chemical changes.
When the composition of the dry nitrogen gas filled up inside the housing is varied in either a short time period or a long time period due to generations of the out gas (containing moisture), the refractive index “n” of the gas between the air gaps in the basic formula (1) of the Fabry-Perot interferometer is varied, so that an error occurs in the wavelength measurement. This variation of the refractive index “n” owns an essentially different factor from the factor which is caused by the changes in the temperatures and pressure of the gas. As a result, even when the error correction for the temperature change and the pressure change is carried out, this error of the above-mentioned wavelength measurement could not be corrected.
As explained above, since the out gas is produced just after the replacement of dry nitrogen gas, the refractive index “n” is varied. As a result, during several days to one week after the gas replacement has been carried out, the wavelength measuring apparatus cannot be utilized. Therefore, the calibration operation of the wavelength measuring apparatus must be postponed until the refractive index “n” becomes stable, and also, the manufacturing steps of the semiconductor devices cannot be executed in higher efficiencies.
Furthermore, depending upon the sort of the out gas, there is another problem that the generated out gas is adhered on the surfaces of the optical components and thus may cause trouble in optical performance.